Neutron tubes

ABSTRACT

A neutron tube or generator is based on a RF driven plasma ion source having a quartz or other chamber surrounded by an external RF antenna. A deuterium or mixed deuterium/tritium (or even just a tritium) plasma is generated in the chamber and D or D/T (or T) ions are extracted from the plasma. A neutron generating target is positioned so that the ion beam is incident thereon and loads the target. Incident ions cause D-D or D-T (or T-T) reactions which generate neutrons. Various embodiments differ primarily in size of the chamber and position and shape of the neutron generating target. Some neutron generators are small enough for implantation in the body. The target may be at the end of a catheter-like drift tube. The target may have a tapered or conical surface to increase target surface area.

RELATED APPLICATIONS

This application claims priority of Provisional Applications Ser. Nos.60/355,576 filed Feb. 6, 2002 and 60/356,350 filed Feb. 13, 2002, whichare herein incorporated by reference.

GOVERNMENT RIGHTS

The United States Government has rights in this invention pursuant toContract No. DE-AC03-76SF00098 between the United States Department ofEnergy and the University of California.

BACKGROUND OF THE INVENTION

The invention relates generally to neutron tubes, and more specificallyto neutron tubes based on plasma ion sources.

Conventional neutron tubes employ a Penning ion source and a single gapextractor. The target is a deuterium or tritium chemical embedded in amolybdenum or tungsten substrate. Neutron yield is limited by the ionsource performance and beam size. The production of neutrons is limitedby the beam current and power deposition on the target. In theconventional neutron tube, the extraction aperture and the target arelimited to small areas, and so is the neutron output flux.

Commercial neutron tubes have used the impact of deuterium on tritium(D-T) for neutron production. The deuterium-on-deuterium (D-D) reaction,with a cross section for production a hundred times lower, has not beenable to provide the necessary neutron flux. It would be highly desirableand advantageous to make D-D neutron sources. This will greatly increasethe lifetime of the neutron generator, and it would greatly reducetransport and operational safety concerns.

Brachytherapy is a type of radiation therapy in which radioactivematerials are placed in direct contact with the tissue being treated.The currently available fast neutron source for brachytherapy treatmentof tumors is a spontaneous fission source such as the radioactiveisotope Cf-252 which is implanted into a patient. All present U.S.Cf-252 neutron source designs require manually afterloaded systems.Because Cf-252 is a radioactive source, it cannot be turned off toprevent excessive exposure of clinical personnel. The average energy ofthe spontaneous neutrons emitted from a Cf-252 source is 2.3 MeV whichis very close to the energy of a D-D neutron source, 2.45 MeV. Byutilizing a D-D neutron tube which can be turned on and off, the patientcan be subjected to radiation treatment at desired times, and clinicalpersonnel will receive no occupational dose from the source while it isturned off during patient preparation.

The utilization of in-situ fast neutrons in treating radioresistanttumors has been demonstrated to be more effective than external neutronsources where the neutrons have been slowed down while penetrating thebody. Since the dose is delivered to the tumor by fast neutrons, it isnot necessary to inject any drug for the delivery of neutron absorbingboron into the tumor, as is often done to increase the capture of slowneutrons. However, boron can be used to enhance the dose delivery toneighboring metastases. Therefore, another advantage of a fast neutronbrachytherapy source over an external source is its capability oftailoring the dose distribution around the region of the tumor.

Therefore, a miniaturized inplantable neutron generator design adaptedfor brachytherapy would be highly advantageous.

It would also be desirable, in many other applications such as cargoscreening, airport luggage screening, and explosives detection, to havea sealed tube neutron generator which provides a high neutron flux withlong life operation and with variable source size. The neutron generatorwould overcome many of the shortcomings of the presently availableneutron tubes.

SUMMARY OF THE INVENTION

The invention is a generic class of neutron tubes or generators based ona RF driven plasma ion source having a quartz or other chambersurrounded by an external RF antenna. A deuterium or mixeddeuterium/tritium (or even just a tritium) plasma is generated in thechamber and D or D/T (or T) ions are extracted from the plasma. Aneutron generating target is positioned so that the ion beam is incidentthereon and loads the target. Incident ions cause D-D or D-T (or T-T)reactions which generate neutrons. The invention may be implemented innumerous embodiments. The general principles and features are the samefor all embodiments which differ primarily in size of the chamber andposition and shape of the neutron generating target.

The invention includes a miniaturized implantable neutron generator ortube that produces fast neutrons from a D-D reaction which can be usedfor brachytherapy applications. The tube is formed of a small RF-drivendeuterium ion plasma ion source and a nearby target to which the ionsare accelerated.

This embodiment provides a small size D-D neutron generator, typicallyless than 8 mm in diameter and 2 cm in length, which allows the sourceto be put right into a tumor. Absorbed dose is delivered by fastneutrons rather than thermal neutrons. The targeted tumor size is lessthan 5 cm in diameter. The dose delivered to the healthy tissue outsidethe tumor is expected to be less than other approaches utilizing anexternal neutron source. Unlike other brachytherapy approaches such asCf-252, there will be zero occupational dose for clinical personnel.

The invention also includes another miniaturized neutron generator ortube that produces fast neutrons from a D-D reaction which can be usedfor brachytherapy applications. The tube is again formed of a smallRF-driven deuterium ion plasma ion source and has a target to which theions are accelerated. However, in this embodiment, the target is at theend of a small diameter drift tube, which can be inserted into the bodylike a catheter. The target also has a tapered or conical shape so thatgreater surface area is provided for higher neutron flux and lowerenergy loading.

The invention includes another embodiment of the neutron generator inwhich the RF driven ion source is substantially larger in size than thefirst two embodiments since it is not designed for implantation in thehuman body. The ion source has a chamber with external RF antenna inwhich a deuterium tritium, or mixed deuterium/tritium ion plasma isproduced. The ions are extracted through a large aperture, typicallysubdivided into a plurality of small apertures, and accelerated to aneutron generating target which has a tapered or conical target surface.The length of the target surface provides a large neutron generatingarea to produce a high flux and reduces beam deposition power density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, partly in section, of a mini-neutron-tubeof the invention.

FIG. 2 is a graph of dose equivalent rate vs. tissue depth for fastneutrons.

FIGS. 3A-D are cross-sectional views of another mini-neutron-tube of theinvention.

FIGS. 4A, B are ion optics computation results using the IGUN simulationcode for double or single gap accelerators.

FIG. 5 is a cross-sectional view of another neutron-tube of theinvention.

FIG. 6 shows a calculated ion beam distribution along a target surfacefor the neutron tube of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a miniaturized implantable neutron generator or“mini-neutron-tube” 10 of the invention, which has a small neutrongenerating target (electrode) 12 closely spaced apart from a smallplasma ion source 14. Plasma ion source 14 is formed of a quartz (orother material) plasma chamber 16. Ion source 14 and target 12 aremounted in neutron generator housing 22.

The general principles of plasma ion sources are well known in the art.Ion source 14 includes an RF antenna (induction coil) 20 surrounding(outside) plasma chamber 16 for producing an ion plasma from a gas,typically deuterium, which is introduced into chamber 16 through aninlet 24 which passes through a vacuum plug 26 at an inlet end ofchamber 16. Antenna 20 is connected to an external RF power source (notshown) through conductors 32 which pass through the end of generatorhousing 22. For neutron generation for brachytherapy applications, theplasma produced in ion source 14 is preferably a deuterium ion plasma.

Ion source 14 also includes extraction electrode 18 at its outlet end.Electrode 18 electrostatically controls the passage of ions from theplasma out of plasma chamber 16.

Spaced apart from ion source 14, and separated therefrom by a highvoltage insulator 28, is target 12. Target 12 is the neutron generatingelement. Ions from plasma source 14 pass through electrode 18 andimpinge on target 12, typically with energy of 80 keV, producingneutrons as the result of ion induced reactions. The target 12 is loadedwith D atoms by the beam. Titanium is not required, but is preferred fortarget 12 since it improves the absorption of these atoms.

In operation, target 12 is biased negatively, e.g. −80 kV, with respectto the extraction electrode 18, which is at ground potential. The biasvoltage is applied to target 12 by high voltage line 30 which extendsout of housing 22 to an external high voltage source (not shown). Highvoltage insulator 28 allows a high voltage to be placed between target12 and extraction electrode 18 even though they are spaced apart by ashort distance.

The neutron source in its preferred embodiments is a tube that istypically less than 8 mm in diameter and 2 cm in length. It can deliver10⁷ n/s operating at a voltage of 80 kV and a beam current of 100microamps. The source has a very simple design. The plasma is generatedby RF induction discharge with the antenna located outside the sourcechamber. Very low RF power, typically less than 30 W, will be employedin the operation of the plasma source. The source and plasma electrodeare biased at ground potential while the target is at a high negativepotential, typically about 80 kV. The deuterium ions will be extractedfrom a small, typically 1 mm diameter, aperture in the plasma electrode,and will be accelerated to the Ti target at 80 kV. Deuterium atoms willbe loaded on the target surface by the beam ions and neutrons will beproduced by D-D reactions.

Because of the small tube size, sufficient neutron flux produced, andfast neutron energy generated (2.45 MeV from D-D reactions) in thisneutron generator, the mini-neutron-tube is well adapted forbrachytherapy applications. The neutrons produced will go out from theend of the neutron generator in virtually all directions so that allparts of a surrounding tumor are irradiated. FIG. 2 shows the doseequivalent rate vs. tissue depth for the source. As can be seen, most ofthe neutron dose is delivered to the tumor itself or the surroundingarea of metastases. Very little of the neutron dose is delivered to thehealthy tissue around the tumor.

FIGS. 3A-D show a miniaturized implantable neutron generator or“mini-neutron-tube” 40 of the invention, which has a small neutrongenerating target (electrode) 42 spaced apart from a small plasma ionsource 44. Plasma ion source 44 is formed of a quartz (or othermaterial) plasma chamber 46, and is similar to plasma chamber 16 shownin FIG. 1. The RF plasma source 44 typically has an outer diameter ofabout 2 cm and is about 2.5 cm in length. Chamber 46 is preferably madeof quartz and is surrounded with a coil or antenna 47, typically made ofcopper. Antenna 47 is connected to an RF source, e.g. a 13.5 MHz RFgenerator, through an impedance matching network (as in FIG. 5). On topof RF antenna coil 47 is another coil 48, typically made of copper,which is connected to a DC power supply. DC current passes through thisouter coil to produce a solenoid B-field for plasma confinement withinchamber 46. The presence of this B-field can lower the operatingpressure within chamber 46 substantially.

Chamber 46 is filled with gas through inlet 49. Chamber 46 is filledwith deuterium gas to produce D⁺ ions for D-D neutron production, or amixture of 50% deuterium and 50% tritium to produce D⁺ and T⁺ ions forD-T neutron production.

The D⁺ or D⁺ and T⁺ ions produced in the ion source chamber 46 are thenextracted and accelerated by extraction and accelerator column 50 whichis formed of a plurality of spaced electrodes 52 separated by insulators53. The first or extraction electrode 52 has a small hole, e.g. 1 mmdiameter, for extraction of the ions, and the additional electrodes 52have aligned apertures. The ions are accelerated to about 100 kV in ashort distance, typically about 3 cm. Extraction and acceleration of thebeam 54 is generally achieved using either a single gap or double gapaccelerator column.

The accelerated beam 54 then enters a long narrow drift tube 56 with atypical diameter less than about 5 mm and a typical length of about 10cm. A cone shaped neutron generating target 42, preferably made oftitanium is mounted in the end of drift tube 56. Drift tube 56 may beformed of spaced concentric outer tubing 57 and inner tubing 58, whichdefine a coolant flow channel 59 therebetween. Water or other coolant isflowed into coolant channel 59 through inlet 61 and removed throughoutlet 62. As shown in FIG. 3B, a pair of spacers 63 divide the channel59 into an inlet channel 64 and outlet channel 65 so that coolant can beflowed along the length of the drift tube 56. FIG. 3C shows the detailsof the tip of drift tube 56 with the target 42 mounted therein, with thecoolant channel 59 passing around the tip. FIG. 3D shows an alternatesingle tube structure for drift tube 56, in which cooling is done by acooling channel 60 at the junction of drift tube 56 with theacceleration column 50.

Target 42 has a cone shaped surface 43 which receives the ion beam 54and becomes loaded with deuterium or a mixture of deuterium and tritium.Ion beam 54 is sufficiently nondiverging so that the beam strikes target54 without substantial loss of ions from the drift tube 56. The conicalsurface 43 of target 42 provides larger surface area for capturing theincident ions. Neutrons are produced on the target surface by D-D or D-Tfusion reactions. If the beam current is 1 mA, the D-D neutron flux willbe 10⁸ n/s and the D-T neutron flux will be 10¹⁰ n/s.

FIGS. 4A, B are ion optics computation results using the IGUN simulationcode for double or single gap accelerators. The results show that theion beam can propagate into the target without impinging on the innerwall of the drift tube. The beam spreads to about 2 mm diameter when itarrives at the cone shaped target and the power density is much reduced.The heat load of about 100 W on the target will be removed bycirculating water. Either of the two water cooling schemes shown abovecan be used, i.e. the double layer tubing arrangement of FIGS. 3A-C orthe edge cooling arrangement of FIG. 3D.

The neutron generator 40 is well suited for brachytherapy since the tipof drift tube 56 with neutron generating target 42 can be inserted indirect contact with the tissues being treated by inserting the tube 56as a catheter. The neutron source can be readily turned on and off asneeded to deliver the desired dosage by simply turning the ion source 44on and off by applying suitable pulses of RF to antenna 47. The Titarget 42 and drift tube 56 are biased at ground potential while theplasma source 44 (or the first electrode 52 of acceleration column 50)is biased at +100 kV relative to ground. The plasma source andaccelerator column are both shielded with a grounded casing (not shown).Thus the part of the instrument in contact with the patient is always atground potential and therefore will post no high voltage danger duringtherapy treatment.

FIG. 5 illustrates another sealed tube neutron generator 70 of theinvention that is based on a RF driven plasma ion source 71 withexternal antenna 72. Ion source 71 is formed of a quartz (or othersuitable material, e.g. ceramic) chamber 73 with antenna (coil) 72 woundexternally thereon. Antenna 72 is connected through matching network 74to RF generator 75 for producing a plasma in chamber 73. While ionsources 10, 40 previously described are generally miniaturized becauseof their medical applications, ion source 71 may be of any size,depending on the application, e.g. about 10 cm diameter; however, theprinciples of operation are similar.

For neutron generation, the plasma is a deuterium or deuterium andtritium (or even just tritium) plasma. The ion source can be operated atseveral mTorr of deuterium or tritium or a mixture thereof. The lowpressure operation enables the design of the accelerator column for highvoltage (e.g. 120 kV) standoff.

Ions from plasma chamber 73 are extracted through plasma electrode 76,which has a large aperture 77 (e.g. >3 cm diameter) which is subdividedinto a plurality of smaller apertures 78 (e.g. 2 mm diameter). Thismulti-beamlet design provides large extraction area with high currentdensity, e.g. 100 mA/cm² or higher) and high atomic ion species (e.g.>90%).

The accelerator column 79 is a single gap column with the neutrongenerating target 80 forms the second or extraction electrode. Neutrongenerating target 80 has a conical or tapered inner surface 82 orientedalong the beam axis to provide a large target area for the ion beam.FIG. 6 shows a calculated ion beam distribution along a target surface.Target 80 is preferably made of titanium and becomes loaded withdeuterium and/or tritium so that neutrons are generated by D-D or D-t(or even T-T) reactions. The length and slope of the conical surface 82can be tailored to minimize the beam deposition power density. Coolingchannels 83 can also be included in target 80 for removing heat. Sincethe length of the target 80 can be extended, generator 70 can provide aline source of neutrons.

The ion source 71 can be operated at high voltage and the target 80 atground potential, e.g. plasma chamber 73 and plasma electrode 76 areshown connected to a high voltage supply (H.V.). Target 80 is enclosedin a metal housing 84, and connected thereto by mount 86. Housing 84 isseparated from HV by insulator 85, and is preferably grounded.Alternatively the ion source can be grounded and target 80 at highvoltage. Housing 84 can also be surrounded by shield 87.

This embodiment of the invention can produce D-D neutron flux higherthan 10¹¹ n/s with modest length and diameter. D-T neutron output isabout two orders of magnitude higher. This neutron generator can form aline source with low beam power density and high neutron flux. Thisconfiguration has particular application for cargo or luggage screeningand for reactor start-up.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

1. A neutron generator including an electrical ground connected to aneutron generating target, said neutron generator comprising: a plasmachamber disposed in a plasma ion source and an external RF antennadisposed outside and around said plasma chamber; an extraction andacceleration system including three electrodes, a first electrode havinga centered aperture being disposed at an end of said plasma chamber forextracting an ion beam, a second electrode having a centered aperturebeing disposed a distance from said first electrode that is between saidfirst electrode and a neutron generating target positioned outside saidplasma chamber, and a third electrode having a centered aperture beingdisposed a distance from the second electrode that is between saidsecond electrode and said neutron generating target, all three aperturesaligned for extracting the ion beam from the plasma chamber, the ionbeam being a tritium, or deuterium and tritium ion beam; said neutrongenerating target electrically connected to ground, said neutrongenerating target including a tapered conical concave surface having anapex, the concave surface having a target surface area into which ionsin the ion beam impact, the concave target surface area being shaped anddisposed so that the ions impact the target surface area at angles fromnormal to the target surface area that are greater than zero degrees,and said neutron generating target being disposed outside and away fromsaid plasma chamber and positioned so that the extracted ion beam isincident thereon to load the target and generate neutrons by reactions.2-3. (canceled)
 4. The neutron generator of claim 1 wherein theacceleration system accelerates the ions to an energy of the order ofabout 100 keV.
 5. The neutron generator of claim 1 wherein the plasmaion source chamber is made of quartz or ceramic.
 6. The neutrongenerator of claim 1 wherein the target is close to the ion source. 7.The neutron generator of claim 6 wherein the generator has a length ofabout 2 cm or less and a diameter of about 8 mm or less.
 8. The neutrongenerator of claim 1 wherein the target is separated from the ion sourceby a drift tube.
 9. (canceled)
 10. The neutron generator of claim 8wherein the ion source has a diameter of about 2 cm or less and a lengthof about 2.5 cm or less, and the drift tube has a diameter of about 5 mmor less and a length on the order of about 10 cm.
 11. The neutrongenerator of claim 8 wherein the drift tube has a cooling channel. 12.(canceled)
 13. The neutron generator of claim 1 wherein the target hascooling channels.
 14. The neutron generator of claim 1 wherein the ionsource chamber has a diameter of about 10 cm.
 15. The neutron generatorof claim 14 wherein the extraction system is a multibeamlet extractionsystem for extracting ion current densities of at least about 100mA/cm².
 16. The neutron generator of claim 1 wherein the ion source isoperated at a pressure of several mTorr.
 17. The neutron generator ofclaim 1 wherein the target is sufficiently long to form a line source.18. (canceled)
 19. The neutron generator of claim 1 further comprisingan RF generator and a matching network through which the RF antenna isconnected to the RF generator.
 20. The neutron generator of claim 1further comprising a plasma confinement solenoid surrounding the RFantenna.